Fault Compartmentalization in a Mature Clastic Reservoir: An Example from Elk Hills Field, California
Mature and aging clastic-dominated hydrocarbon fields commonly become increasingly difficult to produce, causing lower economic return than initially forecast. A major cause of this reduced economic viability is compartmentalization, defined as limitation on the ability to produce hydrocarbons resulting from permeability barriers within a field. Three primary causes of compartmentalization are (i) structural variations in permeability, (ii) stratigraphic variations in permeability, and (iii) permeability reduction resulting from compaction adjacent to producing wellbores. Recognition and delineation of compartmentalization permits formulation of development and depletion plans to maximize recovery and economic value. Here we examine one of several reservoir-scale faults that compartmentalize a portion of the Eastern Shallow Oil Zone, Elk Hills field, California. Using well log, stratigraphic, structural, and pressure data, we apply standard fault seal analyses to the selected fault. Results are compared with known pressure conditions across the fault, and show the fault capable of supporting pressure differentials two to three times those expected from standard fault seal measures. Reasons for this apparently anomalous behavior include (i) reduction of pore fluid pressure resulting from hydrocarbon production over the last 90 years has reduced the slip tendency of all faults in the field, and concomitantly increased their sealing potential; (ii) fault zones contain structures that may not seal the fault over millions of years, but create permeability barriers at production time scales; and (iii) the presence of uninterpreted small-displacement faults adjacent to mapped faults that contribute to permeability reduction - important at production, but not trap formation, time scales.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California